linux-kernel - Re: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in iotlb_sync

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Message-ID: <2fd6f316-0a20-428e-9852-50ba7fb3dae4@linux.intel.com>
Date: Mon, 8 Apr 2024 10:57:32 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: "Tian, Kevin" <kevin.tian@...el.com>, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, "Zhang, Tina" <tina.zhang@...el.com>,
 "Liu, Yi L" <yi.l.liu@...el.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 06/12] iommu/vt-d: Use cache_tag_flush_cm_range() in
 iotlb_sync_map

On 4/8/24 10:51 AM, Tian, Kevin wrote:
>> From: Baolu Lu<baolu.lu@...ux.intel.com>
>> Sent: Sunday, April 7, 2024 2:41 PM
>>
>> On 3/28/24 3:48 PM, Tian, Kevin wrote:
>>>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> Sent: Monday, March 25, 2024 10:17 AM
>>>>
>>>> The iotlb_sync_map callback is called by the iommu core after non-
>> present
>>>> to present mappings are created. The iommu driver uses this callback to
>>>> invalidate caches if IOMMU is working in caching mode and second-only
>>>> translation is used for the domain. Use cache_tag_flush_cm_range() in
>> this
>>>> callback.
>>>>
>>>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>>>> ---
>>>>    drivers/iommu/intel/iommu.c | 22 +---------------------
>>>>    1 file changed, 1 insertion(+), 21 deletions(-)
>>>>
>>>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>>>> index 1c03d2dafb9d..2dcab1e5cd4d 100644
>>>> --- a/drivers/iommu/intel/iommu.c
>>>> +++ b/drivers/iommu/intel/iommu.c
>>>> @@ -1504,20 +1504,6 @@ static void iommu_flush_iotlb_psi(struct
>>>> intel_iommu *iommu,
>>>>    		iommu_flush_dev_iotlb(domain, addr, mask);
>>>>    }
>>>>
>>>> -/* Notification for newly created mappings */
>>>> -static void __mapping_notify_one(struct intel_iommu *iommu, struct
>>>> dmar_domain *domain,
>>>> -				 unsigned long pfn, unsigned int pages)
>>>> -{
>>>> -	/*
>>>> -	 * It's a non-present to present mapping. Only flush if caching mode
>>>> -	 * and second level.
>>>> -	 */
>>>> -	if (cap_caching_mode(iommu->cap) && !domain->use_first_level)
>>>> -		iommu_flush_iotlb_psi(iommu, domain, pfn, pages, 0, 1);
>>>> -	else
>>>> -		iommu_flush_write_buffer(iommu);
>>>> -}
>>> iommu_flush_write_buffer is for a quite different issue. it's clearer to
>>> keep it separated from the iotlb helpers.
>> The VT-d spec describes the write buffer flushing in section 6.8. It
>> states,
>>
>> 1. Write buffer flushing is required only for earlier VT-d hardware
>>      implementations. Newer hardware implementations are expected to NOT
>>      require this.
>> 2. Write buffer flushing is related to cache invalidation. The hardware
>>      performs an implicit write-buffer-flushing as a pre-condition to
>>      the invalidation operations.
>>
>> According to this, write buffer flushing is only required in the map
>> path when IOMMU is not in caching mode and the RWBF iommu capability is
>> set, which perfectly matches the case of cache_tag_flush_cm_range().
>>
>> By doing it in cache_tag_flush_cm_range(), the code will be simpler and
>> easier for maintenance.
>>
> but above applies actually when caching mode is false or when 1st
> stage is being used. this is the opposite of what flush_cm_range()
> implies.
> 
> if we do it through a general flush_range() helper which accepts
> a parameter to indicate the map path, then it's reasonable to move
> this special handling to that helper as that is another trick which
> the caller doesn't need to know.

This could be an option. How about my proposal in another reply?

Best regards,
baolu